The present invention relates to the use of water-soluble disulfide compounds in the preparation of monodisperse regular (non-tabular) grain light-sensitive silver halide emulsions.
Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog can be defined as a developed density that is not associated with the action of the image-forming exposure, and is usually expressed as xe2x80x9cD-minxe2x80x9d, the density obtained in the unexposed portions of the emulsion. Density, as normally measured, includes both that produced by fog and that produced as a function of exposure to light. It is known in the art that the appearance of photographic fog related to intentional or unintentional reduction of silver ion (reduction sensitization) can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, spectral/chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions.
The control of fog, whether occurring during the formation of the light-sensitive silver halide emulsion, during the spectral/chemical sensitization of those emulsions, during the preparation of silver halide compositions prior to coating on an appropriate support, or during the aging of such coated silver halide compositions, has been attempted by a variety of means. Mercury-containing compounds, such as those described in U.S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665, have been used as additives to control fog. Thiosulfonates and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, have also been employed. Organic dichalcogenides, for examples the disulfide compounds described in U.S. Pat. Nos. 1,962,133; 2,465,149; 2,756,145; 2,935,404; 3,184,313; 3,318,701; 3,409,437; 3,447,925; 4,243,748; 4,463,082; and 4,788,132 have been used not only to prevent formation of fog but also as desensitizers and as agents in processing baths and as additives in diffusion transfer systems. More recently, U.S. Pat. No. 5,364,754 describes the use of primarily water-insoluble organic dichalcogenides during the preparation of silver halide crystals to modify the grain growth of silver hlaide crystals in addition to preventing formation of fog without a concomitant large loss in sensitivity. U.S. Pat. No. 5,418,127 describes the use of certain water soluble disulfide compounds which provide the combined attributes of high water solubility and strong antifogging activity with de minimus impact on sensitivity.
A problem has been discovered when attempting to employ some of the water soluble disulfide compounds described in U.S. Pat. No. 5,418,127 in the preparation of monodisperse conventional regular grain silver halide high bromide emulsions. Regular grains are three-dimensional (non-tabular) grains which are substantially internally free of crystal plane stacking faults. Specifically, it has been discovered that the presence of some of such compounds during the precipitation of such emulsion grains lead to the formation of a significant fraction of large grain contamination, generally due to formation of some larger tabular grains. It would be desirable to provide a process for the preparation of monodisperse regular grain emulsions which would enable the use of water soluble disulfide compounds in order to take advantage of the combined attributes of high water solubility and strong antifogging activity with only minimal impact on sensitivity, while avoiding the disadvantages of large grain contamination when emulsion grain size monodispersity is desired.
This invention provides a method of making a monodisperse regular grain silver halide high bromide emulsion comprising precipitating regular silver halide grains comprising greater than 50 mole percent bromide, based on total silver, in a reaction vessel, and adding to the reaction vessel before or during precipitation a disulfide compound represented by the following formula (I): 
where Y and Z are H atoms or substituents, which may be connected to form a carbocyclic or heterocyclic ring; X is xe2x80x94Oxe2x80x94, xe2x80x94NHxe2x80x94 or xe2x80x94NRxe2x80x94, where R is a substituent; M is xe2x80x94Hxe2x80x94 or a cationic species; and L is a linking group, where p is 0 or 1. In preferred embodiments, Y and Z are connected to form a further substituted or unsubstituted phenyl ring.
This invention further provides a silver halide emulsion prepared by the method, and a photographic element comprising a silver halide emulsion prepared by the method. The use of water soluble disulfide compounds of the above formula in the preparation of monodisperse high bromide emulsion grains in accordance with the invention provides the antifoggant advantages of the general class of compounds described in U.S. Pat. No. 5,418,127 in a monodisperse emulsion, while avoiding the problem of large grain contamination in such emulsions.
The disulfide compounds employed in accordance with the invention are represented by following formula (I): 
Y and Z are H atoms or substituents, which may be connected to form a carbocyclic or heterocyclic ring. In preferred embodiments, Y and Z are connected to form a further substituted or unsubstituted phenyl ring. X is independently an xe2x80x94Oxe2x80x94, xe2x80x94NHxe2x80x94 or xe2x80x94NRxe2x80x94. Most preferably X is xe2x80x94NHxe2x80x94. L is a linking group, and p is 0 or 1. In particularly preferred embodiments, the disulfide compound employed is of the formula (II): 
Substituents represented by Y, Z and R, as well as further substituents which may be present Y and Z connect to form a further substituted phenyl ring, are selected from those which do not interfere with the intended function of the disulfide compound in the photographic emulsion and which maintains the water solubility of the compound. Examples of suitable substituents include alkyl groups (for example, methyl, ethyl, hexyl), fluoroalkyl groups (for example, trifluoromethyl), aryl groups (for example, phenyl, naphthyl, tolyl), sulfonyl groups (for example, methylsulfonyl, phenylsulfonyl). Preferred are simple alkyl groups and simple fluoroalkyl groups.
Preferably L is a unsubstituted alkylene group and is usually xe2x80x94(CH2)nxe2x80x94 where n ranges from zero to 11 and is preferably 1 to 3. Other examples of L are given below: 
M is either a hydrogen atom or a cationic species if the carboxyl group is in its ionized form. The cationic species may be a metal ion or an organic ion. Examples of organic cations include ammonium ions (for example, ammonium, tetramethylammonium, tetrabutylammonium), phosphonium ions (for example, tetraphenylphosphonium), and guanidyl groups. Preferably M is hydrogen or an alkali metal cation, with a sodium or potassium ion being most preferred.
Examples of disulfide compounds of formula (I) which may be employed in accordance with this invention include compounds I-A through I-G shown below. 
The solubilized disulfides employed in the invention are easily prepared using readily available starting materials. Most of the solubilized disulfides can be obtained by reacting aminophenyl disulfide or hydroxyphenyl disulfide with the appropriate cyclic anhydride followed by conversion of the free diacid to its anionic form using materials such as sodium bicarbonate. Other solubilized disulfides could be obtained by reacting aminophenyl disulfide or hydroxyphenyl disulfide with the mono chloride of a dicarboxylic acid mono ester, followed by hydrolysis of the ester to the carboxylic acid. Representative synthetic examples are provided in U.S. Pat. No. 5,418,127 referenced above, the disclosure of which is incorporated herein in its entirety. Solubilized non-phenyl disulfides can similarly be obtained by employing analogous non-phenyl reactants.
The method of the invention is specifically directed towards the preparation of monodisperse regular grain high bromide silver halide emulsions. The term monodisperse is intended to refer to emulsions having a coefficient of variation (COV) of grain size of less than 25% (preferably less than 20%), where COV is defined as 100 times the standard deviation ("sgr") of grain size divided by the average grain size. Regular grains are conventional three-dimensional (non-tabular) grains which are substantially internally free of crystal plane stacking faults. The term xe2x80x9chigh bromidexe2x80x9d is intended to refer to emulsion grains comprising at least 50 (preferably 70 and optimally 90) mole percent bromide, based on silver, with any remaining halide being bromide, chloride, or mixtures thereof. Preferably, at least 0.5 (more preferably 1.0) mole percent iodide may be present in the emulsion grains to provide increased photographic speed. While iodide may be present up to saturation levels, it is preferably limited to less than 20 (optimally less than 10) mole percent, based on silver, with any remaining halide being chloride. Silver bromide, chlorobromide, iodochlorobromide, chloroiodobromide and iodobromide emulsions are contemplated.
In accordance with the invention, emulsion grain monodispersity is improved specifically during the precipitation of high bromide regular grains by the reduction of larger than average size grain fractions. The shape of regular silver halide grains in general tends to vary with composition. Silver chloride grains, for example, are usually cubic (i.e., bounded by 6 {100} crystal faces), while high bromide silver halide grains may be cubic, octahedral (bounded by 8 {111} crystal faces) or cubo-octahedral (bounded by 6 {100} and 8 {111}faces). Monodisperse emulsions of this invention may be generally prepared by precipitating silver halide crystals in a colloidal matrix by methods conventional in the art. The colloid is typically a hydrophilic film forming agent such as gelatin, alginic acid, or derivatives thereof. Monodisperse emulsions are typically prepared by a double-jet precipitation process, wherein silver and halide salt solutions are concurrently run into a reaction vessel. Double-jet precipitation as normally practiced is a batch precipitation process, but may also be performed in a continuous precipitation process wherein a portion of the emulsion formed is being continuously removed. Research Disclosure, Item 38957, Section I. xe2x80x9cEmulsion grains and their preparationxe2x80x9d describes conventional precipitation procedures.
In a typical emulsion precipitation process, at least about 10% of the dispersing medium (water and peptizer) forming the final emulsion is initially placed in a reaction vessel. Up to 100% of the final emulsion volume can be present initially in the reaction vessel, since water and soluble salts can be continuously removed by ultrafiltration, as illustrated by Mignot U.S. Pat. No. 4,334,012. At least 10% of the final peptizer can be initially present in the reaction vessel, but no peptizer is required until grain nucleation occurs. The concentration of peptizer in the dispersing medium during grain nucleation and growth is usually less than 5 wt %, typically 1 to 3 wt %. A minor amount of halide ion (e.g., less than 10% of total halide precipitated) is typically initially introduced into the dispersing medium to insure a stoichiometric excess of halide ion during precipitation. The pH of the dispersing medium is usually in the range of from 2.0 to 9.0, typically 3.0 to 7.0. The strong mineral acids HCl and HNO3 and the bases NaOH and KOH are typically used to adjust pH. Temperatures in the reaction vessel usually range from 20 to 90xc2x0 C., preferably 30 to 80xc2x0 C. A soluble silver salt (e.g., silver nitrate) is introduced through one jet. Concurrently a soluble halide salt (e.g., an ammonium halide, an alkali halide or an alkaline earth halide) is introduced through a second jet. When the pH of the dispersing medium is acid, an ammonium counter ion behaves no differently than an alkali metal counter ion. When the pH of the dispersing medium is alkaline, however, ammonium ion acts as a ripening agent.
In the formation of high bromide grains, the boundary between cubic and octahedral/cubo-octahedral depends, in large part, on the silver ion concentration of the precipitating conditions, generally reported as pAg (xe2x88x92log [Ag+]). Typically, octahedral grains form at a higher pAg than cubic grains. The presence of iodide increases the probability of forming grains with octahedral faces, and shifts the boundary to a lower pAg. At a fixed pAg, the grain shapes are progressively more octahedral as the amount of iodide in the emulsions is increased. See generally xe2x80x9cThe Theory of the Photograph Process,xe2x80x9d T. H. James, ed., 4th Ed., Mac-millan Publishing Co., Inc. (1977) p. 94. The present invention is particularly applicable to the preparation of monodisperse octahedral or cubooctahedral silver halide high bromide emulsions.
The crystals formed in the precipitation step may be chemically and spectrally sensitized, as known in the art. Chemical sensitization of the emulsion employs sensitizers such as sulfur-containing compounds, e.g., allyl isothiocyanate, sodium thiosulfate and allyl thiourea; reducing agents, e.g., polyamines and stannous salts, noble metal compounds, e.g., gold, platinum; and polymeric agents, e.g., polyalkylene oxides. A temperature rise is employed to complete chemical sensitization (heat treatment). Spectral sensitization is effected with agents such as sensitizing dyes. For color emulsions, dyes are added in the spectral sensitization step using any of a multitude of agents described in the art. It is known to add such dyes both before and after heat treatment.
After spectral sensitization, the emulsion is coated on a support. Various coating techniques include dip coating, air knife coating, curtain coating and extrusion coating.
In accordance with the invention, the disulfide compounds are added to the silver halide emulsion reaction vessel either before the start of or during precipitation. They can be added to the reaction vessel containing the aqueous gelatin salt solution before the start of the precipitation; they can also be added during precipitation to the salt solution, the silver nitrate solution, or from a separate jet directly into the kettle. However, there is some indication that greater activity may be achieved if the disulfides are added in the salt solution or directly to the vessel before the start of precipitation. The compounds can be added from the beginning or part-way-through precipitation. Parameters such as temperature, stirring time and other variables for precipitating conventional emulsions are known to those skilled in the art.
When the disulfide compounds are added from an aqueous solution it is advantageous to use the alkaline earth salt of the disulfide. Alternatively, the free acid can be used by co-dissolution with appropriate molar equivalents of base, or the free acid can be prepared as a solid particle dispersion and added to the silver halide emulsion, gelatin-only overcoat, or interlayer. Preferably the disulfide compounds are added as the aqueous solution of the alkaline earth salt. Additionally, the disulfide compounds of this invention can be used in combination with sulfinate salts as described in U.S. patent application Ser. No. 978,539, Klaus and Leonard, filed Nov. 19, 1993.
The optimal amount of the disulfide compound to be added will depend on the desired final result, the type of emulsion, the degree of ripening, the point of addition, the chemical structure, and other variables. In general the concentration of disulfide compound which may be utilized is from about 1xc3x9710xe2x88x929 to about 1xc3x9710xe2x88x922 mol/mol Ag, with 1xc3x9710xe2x88x927 to 1xc3x9710xe2x88x922 mol/mol Ag being preferred and about 1xc3x9710xe2x88x925 to 8xc3x9710xe2x88x924 mol/mol Ag being most preferred.
Combinations of the disulfide compounds may be used. The disulfide compounds may also be used in combination with other antifoggants and finish modifiers.
The disulfide compounds are particularly useful with intentionally or unintentionally reduction sensitized emulsions. As described in The Theory of the Photographic Process, 4th edition, T. H. James, Macmillan Publishing Company, Inc., 1977, pages 151-152, reduction sensitization has been known to improve the photographic sensitivity of silver halide emulsions. Reduction sensitization can be performed intentionally by adding reduction sensitizers, chemicals which reduce silver ions to form metallic silver atoms, or by providing a reducing environment such as high pH (excess hydroxide ion) and/or low pAg (excess silver ion).
During precipitation of a silver halide emulsion, unintentional reduction sensitization can occur when silver nitrate or alkali solutions are added rapidly or with poor mixing to form emulsion grains, for example. Also silver halide emulsions precipitated in the presence of ripeners (grain growth modifiers) such as thioethers, selenoethers, thioureas, or ammonia tend to facilitate reduction sensitization.
Examples of reduction sensitizers and environments which may be used during precipitation or spectrochemical sensitization to reduction sensitize an emulsion include ascorbic acid derivatives; tin compounds; polyamine compounds; and thiourea dioxide-based compounds described in U.S. Pat. Nos. 2,487,850; 2,512,925; and British Patent 789,823. Specific examples of reduction sensitizers or conditions, such as dimethylamineborane, stannous chloride, hydrazine, high pH (pH 8-11) and low pAg (pAg 1-7) ripening are discussed by S. Collier in Photographic Science and Engineering, 23,113 (1979).
Examples of processes for preparing intentionally reduction sensitized silver halide emulsions are described in EP 0 348934 A1 (Yamashita), EP 0 369491 (Yamashita), EP 0 371388 (Ohashi,), EP 0 396424 A1 (Takada), EP 0 404142 A1 (Yamada) and EP 0 435355 A1 (Makino).
The disulfide compounds are also particularly useful with emulsions doped with Group VIII metals such as iridium, rhodium, osmium and iron as described in Research Disclosure, December 1989, Item 308119, published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emswirth, Hampshire P010 7DQ, ENGLAND. It is common practice in the art to dope emulsions with these metals for reciprocity control.
A general summary of the use of iridium in the sensitization of silver halide emulsions is contained in Carroll, xe2x80x9cIridium Sensitization: A Literature Review,xe2x80x9d Photographic Science and Engineering, Vol. 24, No. 6, 1980.
A method of manufacturing a silver halide emulsion by chemically sensitizing the emulsion in the presence of an iridium salt and a photographic spectral sensitizing dye is described in U.S. Pat. No. 4,693,965. The low intensity reciprocity failure characteristics of a silver halide emulsion may be improved, without significant reduction of high intensity speed, by incorporating iridium ion into the silver halide grains after or toward the end of the precipitation of the grains is described in U.S. Pat. No. 4,997,751. The use of osmium in precipitating an emulsion is described in U.S. Pat. No. 4,933,272 (McDugle).
In some cases, when such dopants are incorporated, emulsions show increased fresh fog and a lower contrast sensitometric curve when processed in the color reversal E-6 process as described in The British Journal of Photography Annual, 1982, pages 201-203.
The photographic elements of this invention can be non-chromogenic silver image forming elements. They can be single color elements or multicolor elements. Multicolor elements typically contain dye image-forming units sensitive to each of the three primary regions of the visible spectrum. Each unit can be comprised of a single emulsion layer or of multiple emulsion layers sensitive to a given region of the spectrum. The layers of the element, including the layers of the image-forming units, can be arranged in various orders as known in the art. In an alternative format, the emulsions sensitive to each of the three primary regions of the spectrum can be disposed as a single segmented layer, e.g., as by the use of microvessels as described in Whitmore U.S. Pat. No. 4,362,806 issued Dec. 7, 1982. The element can contain additional layers such as filter layers, interlayers, overcoat layers, subbing layers and the like. This invention may be particularly useful with those photographic elements containing a magnetic backing such as described in No. 34390, Research Disclosure, November, 1992.
In the following discussion of suitable materials for use in the emulsions and elements of this invention, reference will be made to Research Disclosure, December 1989, Item 308119, published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire P010 7DQ, ENGLAND, the disclosures of which are incorporated herein by reference. This publication will be identified hereafter by the term xe2x80x9cResearch Disclosurexe2x80x9d.
The silver halide emulsions employed in the elements of this invention can be either negative-working or positive-working. Examples of suitable emulsions and their preparation are described in Research Disclosure Sections I and II and the publications cited therein. Some of the suitable vehicles for the emulsion layers and other layers of elements of this invention are described in Research Disclosure Section IX and the publications cited therein.
The silver halide emulsions can be chemically and spectrally sensitized in a variety of ways, examples of which are described in Sections III and IV of the Research Disclosure. The elements of this invention can include various dye-forming couplers including but not limited to those described in Research Disclosure Section VII, paragraphs D, E, F and G and the publications cited therein. These couplers can be incorporated in the elements and emulsions as described in Research Disclosure Section VII, paragraph C and the publications cited therein.
The photographic elements of this invention or individual layers thereof can contain, among other things, brighteners (Examples in Research Disclosure Section V), antifoggants and stabilizers (Examples in Research Disclosure Section VI), antistain agents and image dye stabilizers (Examples in Research Disclosure Section VII, paragraphs I and J), light absorbing and scattering materials (Examples in Research Disclosure Section VIII), hardeners (Examples in Research Disclosure Section X), plasticizers and lubricants (Examples in Research Disclosure Section XII), antistatic agents (Examples in Research Disclosure Section XIII), matting agents (Examples in Research Disclosure Section XVI) and development modifiers (Examples in Research Disclosure Section XXI).
The photographic elements can be coated on a variety of supports including but not limited to those described in Research Disclosure Section XVII and the references described therein.
Photographic elements can be exposed to actinic radiation, typically in the visible region of the spectrum, to form a latent image as described in Research Disclosure Section XVIII and then processed to form a visible dye image examples of which are described in Research Disclosure Section XIX. Processing to form a visible dye image includes the step of contacting the element with a color developing agent to reduce developable silver halide and oxidize the color developing agent. Oxidized color developing agent in turn reacts with the coupler to yield a dye.
With negative working silver halide, the processing step described above gives a negative image. To obtain a positive (or reversal) image, this step can be preceded by development with a non-chromogenic developing agent to develop exposed silver halide, but not form dye, and then uniformly fogging the element to render unexposed silver halide developable, and then developed with a color developer. Additionally, the preceding process can be employed but before uniformly fogging the emulsion the remaining silver halide is dissolved and the developed silver is converted back to silver halide; the conventional E-6 process is then continued and results in a negative color image. Alternatively, a direct positive emulsion can be employed to obtain a positive image.
Development is followed by the conventional steps of bleaching, fixing, or bleach-fixing, to remove silver and silver halide, washing and drying.
The following examples are intended to illustrate, without limiting, this invention.